1. Technical Field
This invention relates to a novel process for removing sulfur-containing organic compounds from fuels by oxidative desulfurization.
2. Description of the Related Art
For many years, growing concerns over environmental pollution caused by the presence of sulfur-containing compounds in hydrocarbon-based fuels such as diesel, gasoline, and kerosene has provided impetus for the development of desulfurization technology. A high level of sulfur in fuels is undesirable due to the formation of SOx from the combustion of sulfur-containing compounds. SOx in turn causes acid rain to form, leading to widespread damage to buildings and disturbing delicate balances in the ecosystem. Furthermore, sulfur compounds in fuels poison the noble metal catalysts used in automobile catalytic converters, causing fuel to be incompletely combusted and thus result in the emission of incompletely combusted hydrocarbons, carbon monoxide, nitrogen oxides in the vehicle exhaust, all of which are precursors of industrial smog.
To protect the environment against pollution caused by sulfur, governmental agencies have set up guidelines for petroleum refining companies to limit the level of sulfur in commercial fuels. For example, the United States Environmental Protection Agency (EPA) has recently announced plans to reduce sulfur content of diesel fuels from the current 500 parts per million (ppm) to 50 ppm in 2006.
The industrial removal of sulfur from fuels is generally carried out via the well-established hydro-desulfurization (HDS) process, described for example in the GB Patent 438,354. HDS involves the catalytic treatment of fuel with hydrogen to convert sulfur-containing compounds to hydrogen sulfide, H2S. H2S is in turn converted to elemental sulfur by the Claus process. For low point and middle boiling point distillates, the typical HDS reaction requires relatively severe conditions of about 300° C. to 400° C. and 0.7 to 5 MPa.
It has been found that HDS is less effective in removing certain residual sulfur-containing compounds present in petroleum distillates, particularly heterocyclic sulfur-containing compounds such as thiophenes, benzothiophenes (BT), dibenzothiophenes (DBT), especially DBTs having alkyl substituents on their 4 and/or 6 positions (Ind. Eng. Chem. Res. 2002, 41, 4362-4375), as well as higher homologs of these compounds. One possible reason is that the sulfur atom is sterically hindered by the bulky benzyl groups, thereby making the sulfur atom less accessible to oxidative attack.
Although these heterocyclic sulfur compounds may be removed by optionally increasing the severity of HDS reaction conditions, the onset of other side reactions leading to the formation of coke, degradation of the octane level of the fuel, as well as the accompanying increase in energy and hydrogen consumption, makes the HDS option undesirable from an economic perspective.
Therefore, alternative processes have been developed to further lower sulfur content of fuels through the removal of residual sulfur-containing compounds from processed fuels, while maintaining or improving fuel performance. The term “deep desulfurization” is typically applied to such processes.
In general, deep desulfurization is carried out on fuels which have already undergone HDS and thus have sulfur contents that have been lowered from the initial level of several thousand ppm to several hundred ppm. Deep desulfurization is thus distinguished from conventional HDS in that it the oxidation of sulfur occurs at a sulfur concentration that is by comparison much lower. From the perspective of reaction kinetics, reactions that are first order or higher with respect to the reactant become more difficult to carry out as the concentration of the reactant becomes gradually lower.
One current approach to the deep desulfurization of fuels includes the use of transition metal adsorbents for removing the sulfur compounds, as disclosed in US Patent Application No. 2004/0007506, for example.
Another approach that has been investigated is oxidative desulfurization (ODS), in which fuel is contacted with oxidants such as hydrogen peroxide, ozone, nitrogen dioxide, and tert-butyl-hydroperoxide, in order to selectively oxidize the sulfur compounds present in the fuel to polar organic compounds. These polar compounds can be easily separated from the hydrophobic hydrocarbon based fuel via solvent (liquid) extraction using solvents such as alcohols, amines, ketones or aldehydes, for example.
U.S. Pat. No. 3,847,800 discloses an ODS process in which nitrogen dioxide gas is used as an oxidant to oxidize sulfur-containing compounds in diesel fuel. Methanol and ethanol are subsequently used as non-miscible solvents for extracting the oxidized compounds.
European Patent Application No. EP 0 565 324 A1 discloses a method of recovering organic sulfur compounds from liquid oil. The method involves a pure redox-based process between the sulfur compounds and the oxidant. The liquid oil to be processed is treated with an oxidizing agent, such as ozone gas, chlorine gas, peracetic acid or hydrogen peroxide to oxidize the sulfur compounds in the oil into sulfones or sulfoxides. Subsequently, the oxidized products are separated using a combination of means such as distillation, solvent extraction and adsorption.
The use of gaseous or liquid oxidants such as hydrogen peroxide, ozone, dioxirane and ethylene oxide to convert the sulfur compounds present in fuels into sulfones is also disclosed in U.S. Pat. No. 6,160,193. The oxidants are contacted with fuel in liquid phase, and the oxidized products thus formed are subsequently extracted from the fuel by adding dimethyl sulfoxide to the reaction mixture. According to this patent, when hydrogen peroxide is used as oxidant, metal catalysts can be used to accelerate the decomposition of the hydrogen peroxide to form the reactive oxidizing species. The dimethyl sulfoxide forms an aqueous phase which is separable from the hydrocarbon phase by gravity separation or centrifugation. Oxidation is reportedly carried out at about 30° C. to 100° C. at pressures of about 150 psig (about 12.5 bar) or preferably at a pressure of 30 psig (about 2.5 bar).
U.S. Pat. No. 6,402,940 further discloses a method for the oxidative removal of sulfur using an oxidizing aqueous solution that comprises hydrogen peroxide and formic acid in specific molar ratios. This oxidizing solution is mixed with liquid fuel at temperatures of 50 to 130° C., thereby oxidizing the sulfur compounds into polar compounds. The polar compounds are subsequently removed by simple extraction and phase separation.
Finally, the PCT application WO 03/051798 discloses a method for carrying out ODS in which the fuel and oxidant are contacted in the gas-phase. The fuel is first vaporized and then contacted with a supported metal oxide catalyst in the presence of oxygen. Sulfur is liberated from hydrocarbon molecules in the fuel as sulfur dioxide gas, which is subsequently removed with an ion exchange column.